Flatbed scanning systems are well known in the art. See, for example, U.S. Pat. No. 3,752,558 for an invention by Lloyd. In high speed scanning systems such as precision plotters, printers, and the like, a number of problems are encountered in exactly locating image pixels throughout an object field format, particularly a flat format. The scanning beam, which is moved transversely across an object plane, often has non-constant velocity and must traverse non-uniform path lengths. Errors can thus occur in the direction of the scan (herein called the Y direction) due to optical distortion, scanner speed, projection of the scan onto a flat format, or simply because of vibration of the equipment. Errors can also occur in a direction transverse the direction of the scan (herein called the X direction) due to scan inaccuracies such as facet orthogonality error (i.e., non-orthogonality of the faces of the scanning polygon to a line from the center of rotation), facet radii error (i.e., error of distance of facet from center of rotation), bearing wobble, air turbulence, mechanical tolerances, vibration, table drive errors, and the like; such errors may affect the linearity as well as placement of a line of pixels, and thus affect the vertical resolution of the system. While modern scanning systems may use focused beams of the order of 15.mu.m in diameter and pixels spaced at one half mil (0.013 mm) intervals between centers, mechanical and optical systems typically achieve such resolutions at great expense. Fixed platen systems of the prior art do not provide a basis for economically achieving such resolutions in both X and Y directions.
U.S. Pat. No. 4,661,699, for an invention by Welmers et al., which is hereby incorporated herein by reference, discloses a moving platen scanning system for controlling the scanning of a single beam of modulated coherent light onto a flat object plane using a bi-directional reference scale. The system splits a laser beam prior to modulation and directs an auxiliary, unmodulated, reference beam in parallel with the modulated beam. The unmodulated reference beam can be distinguished from the modulated beam by a small divergence from the modulated beam or by different polarization. The beams are reflected off a rotating polygon and the reference beam is transmitted through the bi-directional reference scale onto a photo detector. This system addresses the need to correct or compensate for errors in both directions in that the data is synchronized to a pixel clock derived from the position of the scanned beam on the bi-directional reference scale.
Scanning systems which have a fixed optical system and require translation of the platen introduce mechanical design difficulties because the platen has high inertia. Furthermore, there are operational disadvantages in that a moving platen scanner is inconvenient to use: fixed platen systems are preferred. Scanning systems which rely on a slight divergence between a modulated beam and a reference beam are sensitive to any local imperfections in every optical surface encountered by the two beams. As fixed size and spacing is reduced, this sensitivity becomes more critical. In all modern high-resolution scanning systems the time taken to scan a whole document is of concern. As pixel size and spacing is reduced, more scan lines are required to cover a given document. Systems that scan every raster line in sequence take a long time to scan the whole document because the time to scan the whole document increases in proportion to the number of raster lines and there is an upper limit on the rotational speed of a spinning polygon scanner.